The typical force balance accelerometer comprises a proof mass assembly that includes a reed suspended from a mounting system between upper and lower stators of a magnetic circuit. An example of such a prior art accelerometer is described in U.S. Pat. No. 3,702,073 and is shown herein (as prior art) in FIG. 1. In this accelerometer, the movable reed 26 is part of a unitary fused quartz structure that includes a flexure element 34 used to connect the reed to an annular support ring 20. The upper and lower surfaces 28 of the reed are plated with gold forming two capacitance plates. In addition, inwardly facing surfaces of the top and bottom stators 10 and 12 serve as corresponding capacitance plates, and together with the adjacent plated surfaces 28 of the reed, comprise two pairs of capacitors. Movement of reed 26 in respect to top and bottom stators 10 and 12 due to an acceleration directed along the acceleration sensitive axis deflects the reed and thus changes the capacitance of these two capacitors.
The upper and lower surfaces 28 of reed 26 also each include a torque coil 30. The torque coils are positioned on opposite surfaces of reed 26, with their centers aligned along a line that is generally normal to surfaces 28 and coincident with the acceleration sensitive axis of the accelerometer. The top and bottom stators 10 and 12 each include a permanent magnet 14 that extends into a cylindrical bore 32 of torque coils 30. An electrical current flowing through torque coils 30 interacts with permanent magnets 14, producing a torque that pivots reed 26 about its flexure element 34.
An acceleration acting along the acceleration sensitive axis produces a pendulous rotation of reed 26 and torque coils 30 with respect to stators 10 and 12. A servo-feedback circuit (not shown) senses the change in capacitance between reed 26 and stators 10 and 12 and produces an electrical current that is applied to torque coils 30 to return reed 26 to its neutral position between the top and bottom stators. The magnitude of this current thus directly corresponds to the magnitude of the acceleration along the acceleration sensitive axis, which initially caused the displacement of reed 26.
Each force balance accelerometer design has a characteristic torque constant that determines a current scale factor of the accelerometer. The permeability of stators 10 and 12, and the magnetic force of permanent magnets 14 greatly affect the torque constant and are highly sensitive to the effects of thermal variation. It is not uncommon for the magnetic circuit assembly to have a positive temperature coefficient that affects the current scale factor causing an error in the range from 100 to 200 ppm/.degree.C. Significant errors can therefore arise in acceleration measurements due to changes in the permeability and magnetic force of the magnetic circuit assembly caused by temperature fluctuations.
Several steps can be taken to minimize the effects of the magnetic circuit assembly temperature coefficient. One of the more obvious solutions is to mount the accelerometer in a temperature controlled environment, e.g., in a temperature controlled oven, so that it experiences almost no change in temperature. Alternatively, if the output signal produced by the accelerometer is supplied to a computer, the affect of temperature on acceleration measurements can be compensated by measuring the temperature of the accelerometer and applying an appropriate correction factor developed by modeling the current scale factor as a function of temperature. Others have compensated for temperature effects by connecting a temperature dependent load resistor which is selected to have approximately an opposite temperature coefficient from that of the magnetic assembly to the output of the accelerometer. The latter approach reduces the affect of temperature variations on the voltage scale factor, i.e., the voltage developed across the temperature compensating load resistor per g of acceleration, but has little affect on the basic current scale factor, and therefore, the temperature compensating load resistor or other compensating load circuit must be changed if the voltage scale factor is changed.
U.S. Pat. No. 4,144,764 discloses a temperature compensated servo amplifier for an electrically damped accelerometer. The reference does not disclose any details concerning the nature of the temperature compensation; however, the temperature compensation is applied in parallel with a resistor connected to ground at the non-inverting input of an amplifier in the servo feedback loop of the system. This temperature compensation is therefore applied to the scale factor of the feedback loop. The amplifier in the servo loop permits the output impedance of the system to be relatively low, thereby providing a voltage output that varies with acceleration as opposed to a varying current output. As a result, selection of the voltage scale factor is limited.
A more complicated solution to the problem of temperature compensation is disclosed in U.S. Pat. No. 4,128,010. This reference discloses a temperature compensated force balance accelerometer that includes a non-inductive temperature sensing winding on a magnet. The winding is connected in a bridge input circuit to an amplifier that produces a current in an auxiliary winding. Current flow in the auxiliary winding augments the fields produced by the permanent magnet so as to maintain a constant magnetic field strength in respect to a torque coil as the temperature changes. This approach thus compensates for a reduction in the strength of the magnetic field produced by the permanent magnet due to a decrease in the permeability of the magnet with increasing temperature by adding magnetic flux produced by the auxiliary winding. However, considerable complexity is thus added to the more conventional design of a force balance accelerometer and a potential source for particle contamination from flaking of wire insulation of the conductor used in the auxiliary winding is thereby introduced into this accelerometer design.
Each of the above-noted solutions to the temperature compensation problem adds to the cost of the accelerometer and/or to the complexity of the acceleration monitoring system. A simpler approach that uses an existing accelerometer design and does not require significant post processing is much more desirable. Accordingly, temperature compensation of the accelerometer's current scale factor inside the servo loop should be accomplished without the need for modeling the affect of temperature on the magnetic circuit assembly, controlling temperature, or providing temperature dependent loads. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiment that follow.